The migration of communications networks from being based upon time-division multiplexing (TDM) to being packet-based necessitates adaptation of the methods and apparatus used for testing such networks.
In a packet-based network, the transfer of information is not based on time-multiplexing of several communication channels, but rather upon statistically-multiplexing packet-based traffic. To deliver traffic data from origin to destination, network elements rely upon addressing schemes found at multiple layers of communication protocols to ensure that traffic data gets delivered. Each packet has a header including addresses enabling packets of a particular message to travel separate paths and be reassembled at the destination. The migration to a packet-based architecture means that traffic data from multiple sources are distinguished by the header address and cannot be located by looking at different timeslots. This is a significant issue from a test and measurement perspective because TDM-based networks permitted single-ended testing, with test data simply being “looped back” unchanged at the far end. In a packet-based network, however, test traffic or data cannot simply be looped back unchanged.
Multiple patent applications have addressed this topic. U.S. Pat. No. 7,085,238 (McBeath), for example, discloses modifying header information of test packets at the far end, specifically by reversing the order of source and destination addresses, so that they can be returned to the originating network apparatus, enabling it to have the same test capability that is available in TDM-based network loopback.
U.S. Pat. No. 7,408,883 (Deragon et al.) and related continuation application US 2008/0304420 (Deragon et al.) each disclose a test apparatus and method for performing single-ended loopback testing in communications systems/networks that are packet-based. The procedure is similar to that disclosed in U.S. Pat. No. 7,085,238 but, according to Deragon et al. their procedure will reverse source and destination addresses that are “level 2” addresses, such as MAC addresses, and/or “level 3” addresses, such as IP addresses.
These methodologies and apparatus are not entirely satisfactory, however, because the measurement obtained by looping back the test traffic at one end of the network link under test will provide only a partial view of the situation. The test may indicate that a problem occurred somewhere during the round trip, but will not be able to identify whether it occurred along the forward path or the return path. Nowadays, some networks are asymmetrical in that the characteristics in one network direction are not the same as the characteristics in the opposite direction, i.e., downstream and upstream directions have different characteristics. It is desirable to be able to differentiate between the characteristics of the forward and return paths.
An instrument capable of differentiating between characteristics of forward and return paths is marketed by EXFO Inc. as the Packet Blazer™ FTB-8510B Ethernet test apparatus. With this test apparatus, it is possible to test an Ethernet circuit in both directions at the same time using a local test apparatus at one end of the path and a remote test apparatus at the other end, with the former controlling the latter, at least partially. Thus, both local and remote test apparatus transmit test traffic/data to each other and each analyzes the test traffic/data it received from the other. Once the tests have been completed, the remote test apparatus transmits its test results for the local-to-remote direction to the local test apparatus which collates them with the test results it derived, itself, for the remote-to-local direction.
While the Packet Blazer™ represents an improvement, it is not entirely satisfactory because some network testing needs to be carried out over a relatively long period of time, for example 24 hours or longer. Some problems that become apparent at the beginning of the testing period might be serious enough to justify aborting the testing until they have been resolved. Using the above-described testing apparatus and methods would not reveal such problems until the end of the lengthy test period, resulting in a significant waste of time.